1. Field of the Invention
The invention relates to an electronic switchgear, preferably operating contact-free, having a presence indicator capable of being externally affected, e.g. an oscillator, a switching amplifier disposed downstream of the presence indicator, an electronic switch, e.g. a transistor, a thyristor or a triac, which can be controlled by the presence indicator via the switching amplifier, a supply circuit for the provision of the supply voltage for the presence indicator and for the switching amplifier and a delay circuit inhibiting activation pulses.
2. The Prior Art
Electronic switchgear of the type basically addressed herein is contact-less and has been increasingly used for approximately twenty years in place of electrical, mechanically activated switchgear with contacts, in particular in connection with electrical or electronic measurement, open or closed loop control circuits. This is true in particular for so called proximity switches, i.e. for electronic switching gear which operates contact-free. An indication is given by means of such proximity switches whether an activating element to which the corresponding proximity switch is sensitive has come sufficiently close to the proximity switch. If an activating element to which the corresponding proximity switch is sensitive, has come sufficiently close to the proximity switch, the presence indicator forming an essential part of the proximity switch reverses the electronic switch and in a switchgear acting as a closing element the nonconducting electronic switch now becomes conducting, while in a switchgear acting as an opener the conducting electronic switch now inhibits. (By means of switch gear of the type under discussion it is also possible to indicate whether a physical quantity of an activating medium to which the switchgear is sensitive has reached a corresponding value.) Therefore, the presence indicator which can be externally actuated is, among others, an essential component of electronic switchgear of the type previously described. For example, an inductively or capacitively actuable oscillator can be provided as a presence indicator; these are inductive or capacitive proximity switches (see, for example, German Non-examined or Examined Published Applications or Patent Nos. 19 51 137, 19 66 178, 19 66 213, 20 36 840, 21 27 956, 22 03 038, 22 03 039, 22 03 040, 22 03 906, 23 30 233, 23 31 732, 23 56 490, 26 13 423, 26 16 265, 26 16 773, 26 28 427, 27 11 877, 27 44 785, 29 43 911, 30 04 829, 30 38 692, 31 20 884, 32 09 673, 32 38 396, 33 20 975, 33 26 440, 33 27 329, 34 20 236, 34 27 498, 35 19 714, 36 05 499 and 36 38 409). A photo resistor, a photo diode or a photo transistor can be provided as presence indicator; in this case these are optoelectronic proximity switches (see, for example, German Nonexamined Published Applications Nos. 28 24 582, 30 38 102, 33 27 328, 35 14 643, 35 18 025 and 36 05 885).
In connection with inductive proximity switches it is true for the oscillator, as long as a metallic part has not yet reached a pre-selected distance, that K.times.V=1, with K=feedback factor and V=amplification factor of the oscillator, i.e. the oscillator oscillates. When the respective metallic part has reached the pre-selected distance, the increasing damping of the oscillator leads to a reduction of the amplification factor V, so that K.times.V&lt;1, i.e. the oscillator ceases to oscillate.
In connection with capacitive proximity switches it is true for the oscillator, as long as an actuating body has not sufficiently increased the capacitance between an actuating electrode and a backplate electrode, i.e. has not reached a preselected distance, that K.times.V&lt;1, i.e. the oscillator does not oscillate. When the actuating body has reached the pre-selected distance, the increasing capacitance between the actuating electrode and the backplate electrode leads to an increase in the feedback factor K, so that K.times.V =.times.1, i.e. the oscillator begins to oscillate.
In both embodiments, inductive proximity switch and capacitive proximity switch, the electronic switch, e.g. a transistor, a thyristor or a triac, is controlled depending on the different states of the oscillator. Optoelectronic proximity switches have a light emitter and a light receiver and are also called light barriers. Differentiation is made between the type of light barrier in which the light emitter and the light receiver are disposed at opposite ends of a monitoring course, and the type of light barrier in which the light emitter and the light receiver are disposed on the same end of a monitoring course and a reflector disposed at the opposite end of the monitoring course reflects the light beam emanating from the light emitter back to the light receiver. In either case the presence indicator reacts when the light beam normally traveling from the light emitter to the light receiver is disrupted by an activating element present in the monitoring course. However, there are also light barriers of the light barrier type previously discussed in which the light beam emanating from the light emitter is only reflected back to the light receiver by a respective activating element.
In the beginning electronic contact-free switchgear was subject to a number of problems, in comparison with electrical mechanically actuated switchgear, namely among others "Provision of a Supply Voltage for the Presence Indicator and the Switching Amplifier", "Design of the Presence Indicator", "Resistance to Short Circuits" and "Activating Pulse Prevention". Addressing these problems and their solutions (as well as other problems and their solutions relating to electronic contact-free switchgear) are, for example, German Non-examined or Examined Published Applications or Patent Nos. (19 51 137, 19 66 178, 19 66 213, 20 36 840, 21 27 956, 22 03 039, 22 03 040, 22 03 906, 23 30 233, 23 31 732, 23 56 490, 26 13 423, 26 16 265, 26 16 773, 26 28 427, 27 11 877, 27 44 785, 29 43 911, 30 04 829, 30 38 102, 30 38 141, 30 38 692, 31 20 884, 32 05 737, 32 09 673, 32 14 836, 32 38 396, 33 20 975, 33 26 440, 33 27 328, 33 27 329, 34 20 236, 34 27 498, 35 19 714, 35 29 827, 36 05 199, 36 05 885 and 36 38 409).
In connection with electronic switchgear, which can be connected via an external conductor with one terminal of a supply voltage source and only via another external conductor with a connection of a consumer, the provision of the supply voltage or supply current for the presence indicator and for the switching amplifier is not without problems, because the supply voltage or the supply current must be provided in the conducting state as well as in the inhibited state of the switchgear.
It is of no consequence whether the provision of a supply voltage or of a supply current is addressed. Here, provision represents derivation from the voltage drop occurring at the switchgear, or from the operating current conducted via the switchgear (conducting state), or from the operating voltage present at the switchgear or from the residual current flowing across the switchgear (inhibited state). Therefore it is of no consequence whether the provision of a supply voltage or the provision of a supply current is addressed, because the presence indicator and the switching amplifier of course require a supply voltage and a supply current.
Based on its operation as switchgear, practically no voltage drop should occur in the switchgear herein discussed in the conductive state and practically no residual current should flow in the inhibited state. However since, if no voltage drop is allowed to occur in the conductive state in switchgear with only two external conductors, no supply voltage for the presence indicator and the switching amplifier could be obtained and, if no residual current is allowed to flow in the inhibited state, no supply current could be obtained, it is true for all electronic switchgear with only two external conductors that in the conductive state a voltage drop occurs and in the inhibited state a residual current flows.
It follows from what has been stated above that the voltage drop and the residual current should be as small as possible, even though in electronic switchgear with only two external conductors a voltage drop occurs in the conductive state and a residual current flows in the inhibited state in a way which is unintentional but necessary for the operation. In the beginning it was stated that, among others, a switching amplifier, placed downstream from the presence indicator, and an electronic switch are associated with the electronic switchgear on which the invention is based, and that the electronic switch can be controlled via the switching amplifier by the presence indicator. The term switching amplifier is to be understood in a general way and encompasses the entire circuit between the signal output of the presence indicator and the control input of the electronic switch, thus the entire signal transmission path between the presence indicator and the electronic switch. Of course, the switching amplifier understood in this manner or the signal transmission path may consist of a plurality of operational units.
The following is true for electronic switchgear of the type discussed here, having an oscillator as presence indicator. When such switchgear is connected to the operating voltage source, the switching amplifier is already operational, without the need for particular steps, when the oscillator is not yet operational. Actually the oscillator requires a certain amount of time, after the voltage supplied to it has reached the required minimum value, to build up oscillations, while the switching amplifier is operational practically immediately after the voltage supplied to it has reached the required minimum value. This has the result that, when such switchgear is connected to the operating voltage source, an effect is simulated, namely damping of the oscillator, and that the switchgear reacts accordingly, i.e. with switchgear acting as a closing element the electronic switch becomes conductive and in switchgear acting as an opener the electronic switch inhibits. The phenomenon described above already was recognized about twenty years ago and has been generally called "Appearance of Activating Pulses". Such activating pulses can be prevented by means of a delay circuit. Consequently a delay circuit preventing activating pulses is also part of the electronic switchgear from which the invention proceeds.
It was stated above that the described activating pulses occur when the presence indicator is in the form of an oscillator. However, activating pulses also occur when the presence indicator is other than an oscillator, namely in all cases where the presence indicator requires a certain length of time to be operational. Electronic switchgear having an oscillator as a presence indicator is always described below. However, what has been said is true in general, namely for all electronic switchgear of the type here discussed having a presence indicator requiring a certain length of time to become operational.
In the devices according to the state of the art the delay circuit which prevents activating pulses was first connected directly to the signal transmission path, between the presence indicator and the electronic switch (see Japanese Utility Model JP-GM 42-2278 and German Examined Published Application DE-AS 20 54 100). This is disadvantageous, because the delay circuit is not only activated when such switchgear is connected to the operating voltage source, but is continuously operational and thus limits the maximum switching cycle of such switchgear. The disadvantage described above was already noted in 1973 and therefore electronic switchgear of the type under discussion was developed in which the delay circuit is placed upstream of the switching amplifier or of an operational element of the switching amplifier in the supply voltage (see German Examined Published Application DE-AS 23 56 490). This assures that the delay circuit is operational when such switchgear is connected to the operating voltage source so that the maximum switching cycle of the switchgear is not influenced. Equivalent thereto is another, also known, solution of the problem "Prevention of Activating Pulses" which is characterized in that an operational unit of the switching amplifier is in the form of an AND gate and in that the presence indicator, directly or by means of at least one additional operational unit of the switching amplifier, is connected to the first input of the AND gate and the delay circuit to the second input of the AND gate.
All solutions of the problem "Prevention of Activating Pulses" known in the prior art have the disadvantage that under certain conditions they still do no prevent activating pulses, namely, among others, when either the switchgear is repeatedly quickly connected to the operating voltage source, for example because of chattering contacts, or when the operating voltage source supplies a slowly increasing operating voltage.